firefox: set some defaults for the video hw

[brianmcgillion]

Description of Changes

Add some more defaults to enable more offload to the GPU for video rendering.

Type of Change

• New Feature
• Bug Fix
• Improvement / Refactor

Related Issues / Tickets

Checklist

• Clear summary in PR description
• Detailed and meaningful commit message(s)
• Commits are logically organized and squashed if appropriate
• Contribution guidelines followed
• Documentation updated with the commit - https://tiiuae.github.io/ghaf/
• Author has run nix flake check and it passes
• All automatic GitHub Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing Instructions

Applicable Targets

• Lenovo X1 x86_64
• Dell Latitude x86_64
• Alienware x86_64

Installation Method

• Requires full re-installation (just build ...installer)
• Can be updated with just rebuild ...
• Other:

Test Steps To Verify:

1. ...